A solid-electrolyte fuel cell (hereinafter referred to as a fuel cell), which has been developed in recent years as a power source for electrical vehicles, generates electrical power by utilizing the electrochemical reaction of the fuel and oxidant gases. The fuel gas is ionized at the anode and the oxidant gas is ionized at the cathode. The ions of fuel gas (protons, hydrogen ions) travel by means of the solid-electrolyte membrane and react with the oxygen ions at the cathode to produce water, thus generating electrical power.
As one of the parameters governing the efficiency of the power generation of fuel cell, the ion conductivity of hydrogen ion is given, which travels in the solid-electrolyte membrane. The higher the ion conductivity is, the more the electrical power generation resulting from the electrochemical reaction will be, since the number of hydrogen ions which can travel in the solid-electrolyte membrane per time increases. On the other hand, when the ion conductivity is low, the amount of electrical power generation will decrease, since the number of hydrogen ions which can travel in the solid-electrolyte membrane per time decreases.
Several inventions related to raising the ion conductivity of solid-electrolyte membrane have been provided and a humidifying apparatus for fuel cell, for example, is disclosed in the patent gazette Japanese Laid-Open Patent 8-273687.
In the humidifying apparatus according to Japanese Laid-Open Patent 8-273687, the fuel gas is humidified by the cooling fluid of the fuel cell and the drying of the solid-electrolyte membrane is prevented by supplying the humidified fuel gas to the fuel cell. The humidifying apparatus is equipped with the hollow fiber membrane. The fuel gas flows inside the hollow fiber membrane and the water flows outside it. This hollow fiber membrane is capable of separating the liquid phase outside the hollow fiber membrane and the gas phase inside it, and making the permeation of the moisture from the side of liquid phase with higher water vapor partial pressure to the side of gas phase, which has relatively lower water vapor partial pressure. The moisture moved from the side of liquid phase to the side of gas phase through the solid-electrolyte membrane is evaporated by the flow of fuel gas, thus converting the fuel gas into the humidified fuel gas containing a given amount of water vapor.
However, when the moisture contained in the off-gas discharged from the fuel cell is used for the humidification of fuel gas, gas molecules such as oxygen molecules possibly permeate the hollow fiber membrane depending on the type of it, since the off-gas is discharged with the oxidant gas containing the oxygen gas before reaction. If the oxygen gas in the off-gas permeates the hollow fiber membrane and mixes with the fuel gas, the fuel gas containing the oxygen gas will be supplied to the anode. In this case, the fuel and oxygen gases possibly react to create heats with the platinum electrode of the anode as a catalyst before the electrochemical reaction, sometimes causing the degradation of the solid-electrolyte membrane and the platinum electrode. Further when this type of hollow fiber membrane is used, a purge line and a control device are required for purging the gas in the pipes at the starting of fuel cell, since the fuel and oxygen gases possibly mix through the hollow fiber membrane during the prolonged non-operation time period of fuel cell.
The optimization of humidifying means as the total fuel cell system has been desired, like an encouraged humidification of oxidant gas, so as to achieve the high efficiency of power generation by humidifying.